The invention relates to a method and device for producing a surface coating consisting of aggregates and bitumen on a surface such as a road.
Worn or damaged roads may be repaired by producing, on the surface of this road, a surface coating based on aggregates, such as loose chips and bitumen.
The current technique for producing a surface coating includes spraying the road with a thick layer of bituminous binding material (for example a layer of 1.3 to 1.8 kg of bitumen per square meter of road). Then, loose chips, such as crushed rocks, are poured onto the binding material in a quantity which is excessive relative to the quantity required to just cover the road. Finally, compactors are used to ensure the best adhesion possible for the loose chips on the layer of bituminous binding material.
It is necessary to pour, onto the layer of bituminous binding material, a quantity of loose chips which is greater than the quantity required to just cover the layer of binding material so that there is no exposed bituminous binding material remaining to adhere to the tires of vehicles.
Those aggregates which do not adhere to the bitumen are thrown in all directions by vehicles travelling on the road and can cause the windscreens of other vehicles to shatter and paintwork to be chipped as well as, in urban areas, windows to be shattered.
Moreover, the cost of the coating produced in this manner is increased by virtue of the fact that good-quality loose chips are wasted.
This loss of loose chips just after the coating has been laid on the road constitutes what is known as operational rejection.
There is also another type of rejection which is postponed until during the weeks following the laying of the coating and which consists of a progressive tearing away of the poorly adhered loose chips or of those loose chips which have adhered over too small a part of their surface.
Finally, in the first period of cold weather following the laying of the coating, there will be a rejection known as first-winter rejection which occurs due to the fact that the aggregates which adhered satisfactorily when the binding material was still plastic are bound in a fragile manner and, when the first cold spell arrives become brittle. Travelling vehicles remove the small percentage of loose chips whose adhesion has not withstood the first cold spell.
One year after laying of the coating, the loose chips which have withstood all the tests of the traffic under the various weather conditions experienced during the year are henceforth satisfactorily fixed and generally do not detach themselves further except when they are worn and when the adhesive binding material becomes too old.
In order to withstand tearing away, the loose chips which are poured onto the binding material must have as large a surface as possible in contact with the adhesive binding material. Because the crushed loose chips do not have simple geometric shapes, such as the shape of a cube or the shape of a truncated pyramid, a point of a loose chip is often located opposite a face thereof.
When the loose chips are disposed so as to have a point facing upwards, the corresponding disposition affords advantages in that the tires of vehicles grip well in wet weather. On the other hand, this leads to more rapid wear of the tires and to noise being produced due to the contact of the tires with the ridges or points of the loose chips.
When loose chips are poured onto a layer of adhesive bituminous binding material, good and durable adhesion is produced only when the following conditions are fulfilled:
the loose chips must not be dusty or contaminated with soil, and must be dry, which is rarely the case, and, similarly, the surface of the road must be clean and dry;
the adhesive bituminous binding material must be sufficiently fluid to spread and moisten the loose chips, which requires the coating to be produced during a period of sufficiently hot weather. This limits the period during which surface coating can be laid to repair roads, in the geographical zone to which France belongs, to the five warmest months of the year, from May to September.
The coating technique, implemented according to known methods, is thus risky, since it is sufficient for there to be excessive humidity, rain, a cold spell, for dirty loose chips to be used or for a road surface to be contaminated with soil, for the coating produced on the site to be of insufficient quality.
Roads which have to be repaired usually have a defective longitudinal or transverse profile which is impossible to rectify using known techniques for producing surface coatings. In fact, by applying a layer of binding material on a deformed support and then fixing thereto a single layer of loose chips, the initial profile is retained in its general form. This also applies when two or more layers of loose chips are superposed in order to form the coating, the defects being reproduced in each of the successive layers.
These defects are reflected in a lack of comfort when driving vehicles, in particular at the maximum speeds authorized on the road network.
In addition to the above technique for producing coatings, roads are also repaired by depositing layers of bituminous coated products which are bound to the surface of the road by means of a layer of binding material of very small thickness, generally less than 10% of the total quantity of bitumen used. The coated material, which consists of a mixture of bitumen and aggregates of various particle sizes, has the form of a malleable mass which is spread and compacted on the layer of binding material.
The covering obtained is generally very compact and smooth, and the absence of roughness leads to poor tire adhesion for this type of covering, particularly in wet weather. On the other hand, the spreading and compacting of a relatively homogeneous mass of malleable material on an uneven road generally makes it possible to compensate for the small defects in the roads profile when the covering is sufficiently thick.
French Patent 2,550,248 discloses a mobile device for the cold production and spreading on site of bituminous coated products for surfacing roads. In particular, this machine can permit the production and spreading on site of bituminous concrete consisting of a material with a small particle size, such as sand, mixed with an emulsion of bitumen.
This device, which can travel on a road at high speed and on site at low speed, by virtue of a dual transmission, has several possibilities for receiving or storing solid or liquid materials and for processing them. This integrated device, however, has never been set hitherto for producing coatings for repairing a road.